Gated three-dimensional (3-D) cameras, for example time-of-flight (TOF) cameras, provide distance measurements to objects in a scene by illuminating a scene and capturing reflected light from the illumination. The distance measurements make up a depth map of the scene from which a 3-D image of the scene is generated.
Conventional TOF systems ascertain depth distances (Z) to a target object by emitting modulated optical energy of a known frequency, f, and examining phase-shift in the optical signal reflected from the target object back to the TOF system. Exemplary such phase-type TOF systems are described in several U.S. patents received by Canesta, Inc., and now assigned to Microsoft, Inc. Such patents include U.S. Pat. No. 6,515,740 entitled “Methods for CMOS-Compatible Three-Dimensional Imaging Sensing Using Quantum Efficiency Modulation”, U.S. Pat. No. 6,906,793 entitled Methods and Devices for Charge Management for Three Dimensional Sensing, U.S. Pat. No. 6,678,039 “Method and System to Enhance Dynamic Range Conversion Useable With CMOS Three-Dimensional Imaging”, U.S. Pat. No. 6,587,186 “CMOS-Compatible Three-Dimensional Image Sensing Using Reduced Peak Energy”, U.S. Pat. No. 6,580,496 “Systems for CMOS-Compatible Three-Dimensional Image Sensing Using Quantum Efficiency Modulation”.
In practice, changes in Z produce a measurable change in phase shift, but eventually the phase shift begins to repeat, e.g., θ=θ+2π, etc. Thus, distance Z is known modulo 2πC/(2ω)=C/(2f), where f is the modulation frequency. As such, there can be inherent ambiguity between detected values of phase shift, θ, and distance Z.
It is known to disambiguate, or de-alias, the phase shift data by emitting light from the light source at multiple frequencies for each frame of image data. However, this operation results in a high consumption of power and mathematical computation, which may not be practical or optimal for certain TOF systems such as portable depth imaging systems.